The present invention concerns a method of and an apparatus for operating a double-fed asynchronous machine on a mains network upon transient mains voltage changes.
The double-fed asynchronous machine is preferably used in variable-speed systems, for example in high-efficiency wind power installations, as a wave generator or in conjunction with inertia mass storage devices and uninterruptable power supplies. In general the stator of the asynchronous machine is connected to the mains network and the rotor is connected to an inverter by way of slip rings. With such an inverter a target or reference value of an electrical parameter can be impressed on the rotor. The term inverter is to be interpreted broadly. In general in modern installations there is an inverter at the machine side and an inverter at the mains network side, which are connected together by way of an intermediate circuit. Besides voltage and current intermediate circuits, direct inverters are also possible, which manage without an intermediate circuit. Feed for the machine-side inverter is also possible by a dc voltage source or a dc source. The electrical parameter which is impressed on the rotor can be for example a voltage or a current. In general nowadays four-quadrant inverters with IGBTs are used. However other semiconductor switches such as GTOs and thyristors are also possible.
The advantage of the double-fed asynchronous machine over comparable systems lies in the reduced nominal load capacity of the inverter, in relation to the total power which can be fed into the mains network. As a result a system with a double-fed asynchronous machine enjoys comparatively good efficiency.
The amplitude and frequency of the rotor voltage are proportional to the slip of the machine. The slip is defined as the deviation of the mechanical rotary speed from the synchronous rotary speed with respect to the synchronous rotary speed. Typical speed ranges for the double-fed asynchronous machine are between 70% and 130% of the synchronous speed. In that working range in respect of rotary speed the required maximum rotor voltage is considerably lower than when the machine is stationary. The amplitude of the rotor voltage in the stopped condition is more than three times as great as with 30% slip. The absolute value of amplitude depends on the transformation ratio of the machine. The frequency of the induced rotor voltage in the stopped condition is equal to the mains frequency. In the typical speed range the frequency is at a maximum 30% of the mains frequency. The inverter is normally so designed that it can supply at a maximum the required voltage in the defined speed range.
In the course of the increasing number of wind power installations and the demands which are linked thereto and which are becoming ever higher, in respect of the mains network operators, the performance of the double-fed asynchronous machine is of increasing interest, in the case of transient changes in the mains voltage. In that respect transient changes are time-limited deviations in respect of the mains voltage from the steady-state value or from the nominal value. That can be both drops in voltage and also increases in voltage. In general these are called voltage jumps or transient voltage jumps. Drops in voltage can occur in that case for example due to short-circuits in the mains network. In that respect drops in voltage can in the extreme case occur down to 0%. Increases in voltage can occur with a non-compensated reactive power balance in the network, for example when large inductive loads are switched off.
In the case of a double-fed asynchronous machine the stator of the machine is connected directly to the mains network so that, upon a transient change in mains voltage, the stator voltage behaves in a corresponding fashion. The flux vector of the machine rotates in the steady-state condition with the mains frequency. In the case of a transient voltage change that involves a component of the magnetic flux, which is stationary relative to the stator and decreases again only after a number of mains periods. That stationary flux component, also referred to as a direct or steady component of the flux, is proportional to the change in the mains voltage vector. The direct or steady component of the flux induces in the rotor winding a considerably higher voltage than in the steady-state mains network mode of operation with a defined speed range. The inverter however is generally only designed for such a steady-state mains network mode of operation and is therefore not in a position to supply a corresponding counter-voltage. The consequence of this is that the response characteristic on the part of the double-fed system upon voltage jumps exhibits short circuit-like currents in the stator and in the rotor as well as a corresponding air gap torque. The simplest technical solution for an optimum response behavior on the part of the double-fed asynchronous machine would be to design the inverter for a rotor voltage which is necessary for compensation of a maximum mains voltage jump. It will be noted however that that would almost be equal to an inverter nominal load capacity corresponding to the total power of the system. That however nullifies the essential advantage of the double-fed system, namely the comparatively low nominal load capacity of the inverter.
Generally a so-called crowbar is used to protect the inverter of a double-fed asynchronous machine. That is a protective circuit with thyristors, by which the rotor can be short-circuited. The tripping criterion for the crowbar can be the fact of exceeding an admissible rotor current, an admissible intermediate circuit current or an admissible intermediate circuit voltage. As already explained a transient mains voltage change can lead to inadmissibly high rotor currents. Correspondingly the voltage induced in the rotor in the case of transient mains voltage changes can also lead to a feed of energy into the intermediate circuit and thus an increase in the intermediate circuit voltage or the intermediate circuit current. The crowbar admittedly protects the inverter from damage to the intermediate circuit, but the use thereof has serious disadvantages in regard to the overall performance of the double-fed asynchronous machine:
A normal mode of operation, controlled by the inverter, of the double-fed asynchronous machine, for example with regulation of the active and reactive power delivery to a fixed value or regulation on the basis of other parameters, is no longer possible during activation of the crowbar. With the rotor short-circuited the machine acts like an asynchronous machine with a squirrel-cage rotor winding, that is to say the machine receives inductive reactive power from the mains network in dependence on the rotary speed and takes active power from the mains network or delivers same. If in the course of a transient mains voltage change the crowbar is activated then generally at least 100 ms elapses after the end of the change in voltage before a defined mode of operation, for example with active and reactive power regulation, is possible again. That means that the requirements of the mains network operators, for also actively regulating active and reactive power in the case of transient voltage changes, cannot be met. There may also be a requirement that, instead of active and reactive power, other corresponding parameters such as mains voltage, power factor, moment or apparent power are to be regulated.
As already mentioned, the short-circuit-like currents in the case of transient mains voltage changes lead to a corresponding air gap torque. Such a torque loads the drive train and the transmission. The torque acting on the drive train and the transmission is further increased by the use of the crowbar. It is not just the amount of the torque that is critical in that case, but also the alternating components which occur in respect of the torque and which occur to a particular degree in the case of asymmetrical mains network faults. Such torque loadings in the normal case admittedly do not lead to direct damage, but with a corresponding frequency thereof the service life of the transmission and other components of the drive train can be considerably reduced.
WO 2004/030199 describes an apparatus for the continuous feed of energy into the mains network with a double-fed asynchronous machine in the event of abrupt mains voltage changes. The apparatus includes an electronic switch in the stator circuit, by which the stator is temporarily separated from the mains network upon voltage changes. That apparatus suffers from the disadvantage that the machine has to be synchronized with the mains network again.
The publication by A Causebrook, D J Atkinson and A G Jack ‘Fault Ride-Through: Shifting the Balance of Power from Blade Pitch to Electrical Resistance’, Athens, EWEC 27.02.-02.03.2006, discloses an arrangement having an electronic switch and a parallel resistor. In that arrangement in the case of a mains fault the resistor is connected into the mains network path and thus permits electrical energy to be conducted out of the machine. Such an arrangement is really good in limiting the current and torque peaks occurring immediately after the fault occurs. To ensure decay of the time alternating components of current and torque however quite long switch-on times are required for the resistor. The required switch-on times for realistically designed resistors are markedly above 20 ms. That is the time after which energy supply companies require regulated operation with the delivery of defined active and reactive power. With longer switch-on times for the resistor it can further happen that not only active power is taken from the machine, as is desired, but also from the mains network. The latter is even prohibited in some mains connection guidelines.
Besides circuitry solutions, considerations relating to the regulating methods for a double-fed asynchronous machine in relation to performance in the case of transient mains voltage changes are also to be found in the state of the art, thus also in the document by Jorun I Marvik, Torstein Bjorgum, Bjarne I Naess, Tore M Undeland and Terje Gjengedal ‘Control of a Wind Turbine with a Doubly Fed Induction Generator after Transient Failures’, NOEPIE 14.-16.02.2004. Here the performance of a reactive power regulator and a flux regulator are compared together.
WO 2006/030183 discloses a stator voltage and stator power regulation with a subordinated rotor flux regulation for the regulation of the double-fed asynchronous machine in the case of steady-state mains voltage and additional auxiliary regulators for optimizing the performance in the case of transient mains voltage changes.
It is to be seen from the aforementioned state of the art that flux regulation of an electric machine or a double-fed asynchronous machine is known both in the steady-state and also in the transient condition. In that case the reference or target value is generated for the flux to be regulated from a superordinated regulator or is set to a constant value or—in the case of a transient mains voltage change—to a quasi-steady-state value.